1. Field of the Invention
The present invention relates to a technique of evaluating an electrode catalyst fixed to an electrode of a fuel cell, a technique of evaluating the performance of a fuel cell, an electrode catalyst evaluated by the evaluation technique, a method of producing an electrode catalyst, a fuel cell with an electrode catalyst, and a method of manufacturing a fuel cell.
2. Description of the Related Art
The performance of the electrode catalyst is one important factor affecting the performance of the fuel cell. Lots of techniques of evaluating the function of the electrode catalyst have been proposed to evaluate the performance of the fuel cell. One known evaluation technique of the electrode catalyst discharges a manufactured fuel cell and measures the output potential of the fuel cell. Another known evaluation technique utilizes cyclic voltammetry (CV method), which soaks the electrode catalyst in an electrolytic solution and sweeps the potential to measure the electric current. These prior art techniques of evaluating the electrode catalyst adopt the electrochemical method that supplies electricity to the electrode catalyst.
The fuel cell under the vigorous research and development is a laminate of multiple unit cells. Each unit cell is formed by interposing a membrane-electrode assembly (MEA) between a pair of separators. For the desired performance of the fuel cell, each MEA of the cell laminate is required to have the performance of or over a specific level. Evaluation of the electrode catalyst after manufacture of the fuel cell should accordingly be carried out by the unit of MEA. Since the output voltage of each MEA is feeble, meticulous attention is required to prevent adhesion of metal ions. Evaluation of the electrode catalyst accordingly takes significant process and time. Another problem is that manufacture of the MEA is essential for evaluation of the electrode catalyst. Manufacture of the MEA takes time and labor and has difficulty in regulation for the desired properties.
In general, the electrode catalyst for the fuel cell includes a noble metal, for example, platinum, carried on a carbon carrier and is fixed with an electrolyte binder to the electrolyte membrane to form the MEA. Carbon typically has the particle diameter of about 20 to 100 nm, and primary particles of carbon are aggregated to form agglomerate. Platinum, on the other hand, has the particle diameter of about 2 to 3 nm, and the electrolyte binder has the particle diameter greater than the space between adjoining primary particles of carbon. The electrolyte binder can thus not enter the space between the adjoining primary particles of carbon. Part of platinum accordingly does not contribute to the catalytic reaction. In the CV method that soaks the electrode catalyst in an electrolytic solution for measurement of the electric current, the particle diameter of the electrolyte solution is smaller than the space between the adjoining primary particles of carbon. Platinum that is not in contact with the electrolyte binder (that is, platinum that has no contribution to the catalytic reaction) is accordingly exposed to the electrolytic solution. This results in evaluation of the performance of the electrode catalyst including the platinum having no contribution to the catalytic reaction. Namely the evaluated performance of the electrode catalyst (evaluated performance of the fuel cell) may be inaccurate.
The properties of the fuel cell including the electrode catalyst can not be expected at the stage prior to production of the electrode. There is accordingly difficulties in manufacture of the high-performance fuel cell. A large variation in performance among the respective unit cells results in poor properties of a resulting fuel cell stack obtained by laminating multiple unit cells. In order to give the high-performance fuel cell stack, the performance of each unit cell should be evaluated prior to stacking. This lowers the production efficiency.
Such problems are not restrictive in the platinum catalyst or the alloy catalyst, but are common to manufacture of the fuel cell including any electrode catalyst. A platinum-iron alloy catalyst is a known example of the alloy catalyst (see JAPANESE PATENT LAID-OPEN GAZETTE No. 3-68452, for example). No practical techniques have been proposed to simplify and improve evaluation of the performance of such alloy catalysts.